Power converters installed in multilevel switching for use in middle and high tension facilities are known from the state of the art (cf. U.S. Pat. No. 5,737,201 and EP 0 944 163 A1). A multilevel switching permits a modular construction of a power converter. Each of the modules includes at least two semiconductor power switches and at least one capacitor arranged between the power switches. The particular character of multilevel switching is that the capacitors do not all lie at the same potential, but can be related to different potentials (so-called “floating capacitors”). An intermediate circuit voltage is in this way passed on to several “floating capacitors” such that the voltage stress of the semiconductor power switch appears as the difference between the voltages of two capacitors.
With power converters installed in multilevel switching, the energy supply unit (as with power converters with semiconductor power switches connected in series) may furnish the auxiliary energy for a control circuit for activation of various power switches at different high potentials. Even with other auxiliary facilities, such as, for example, measuring circuits, an energy supply at high potential must be guaranteed.
Various energy supply units for energy supply of power converters are known from the state of the art. Thus, for example, a single voltage provision is known in which the energy transmission and a potential separation for the individual semiconductor power switches take place by conventional prefabricated impulse transmitters, whereby the windings of the transmitter are insulated from one another using a casting compound. But it is technically very expensive to manufacture such energy supply units for such high insulation voltages as they occur with control switches for medium and high voltage facilities. These known energy supply units are correspondingly expensive.
Moreover, energy supply units with a common voltage supply through a medium frequency (MF) ring feeder are known. The MF ring feeder is fed from a centrally arranged MF converter with medium frequency alternating current in the range of some 10 kHz. The MF ring feeder includes an insulated high voltage (HV) cable that runs in the power converter which leads to a relatively high leakage inductance. Energy tapping at semiconductor power switch drivers located at various electric potentials is undertaken with the aid of ferrite cores that envelop the HV cable. The potential separation, moreover, must either completely take possession of the insulation of the HV cable upon which correspondingly high standards or glow dielectric strength are then to be imposed, or the air space between cable jacket and ferrite core must be sufficiently amply dimensioned to avoid flashovers to the core material reliably.